Structural Characterization and High-Temperature Behavior of Silicon Oxycarbide Glasses Prepared from Sol-Gel Precursors Containing Si-H Bonds

Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000°C of gel precursors obtained by cohydrolysis of triethoxysilane, HSi(OEt)₃, and methyl-diethoxysilane, HMeSi(OEt)₂. The oxycarbide structures have been carefully characterized by means of different techniques such as ²⁹Si magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500°C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano-sized β-SiC crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500°C for the samples in which the presence of Si-Si bonds was postulated at 1000°C.     

Preparation of Silicon Carbide and Aluminum Silicon Carbide from a Montmorillonite–Polyacrylonitrile Intercalation Compound by Carbothermal Reduction

Both silicon carbide and aluminum silicon carbide have simultaneously been obtained directly from naturally occurring aluminosilicate by carbothermal reduction for the first time. A precursor of a montmorillonite–polyacrylonitrile (PAN) intercalation compound was heated at 1700°C in Ar. For comparison, montmorillonite–carbon mixtures were similarly heated. α-SiC, β-SiC, and Al₄Si₂C₅ formed from the montmorillonite–PAN intercalation compound. Mainly α-Al₄SiC₄ was obtained with ternary carbides from the montmorillonite–carbon mixtures in addition to a large amount of β-SiC. Hence, aluminum silicon carbide formation was affected by the mixing condition of the starting materials.     

Influences of Zirconia and Silicon Nucleating Agents on the Devitrification of Li2O · Al2O3· 6SiO2 Glasses

The effects of Si and ZrO₂ dopants on the crystallization and phase transformation process in Li₂O · Al₂O₃· 6SiO₂ glasses were investigated using differential thermal analysis, X-ray powder diffractometry (XRD), and high-resolution transmission electron microscopy (TEM) interactively. Phase separation was observed in the studied glasses prior to substantial crystallization. Elemental Si (1 mol%) significantly aided in glass devitrification. Dropletlike phase-separated regions in the as-quenched or heat-treated glass devitrified at ∼760°C, which in turn provided sites for the heterogeneous nucleation and growth of β-quartz(ss) (solid solution), which transformed to β-spodumene(ss) at higher temperature. Low-temperature surface crystallization in these glasses occurred as low as 760°C. ZrO₂ has limited solubility in this glass system. Small ZrO₂ crystallites (·5 nm) in the as-quenched glass acted as sites for the heterogeneous nucleation and subsequent growth of large (<5 μm) β-quartz(ss) crystals in glasses containing 1.0 mol% or more ZrO₂. The transformation from β-quartz(ss) to β-spodumene(ss) was increasingly inhibited with ZrO₂ additions. The nucleating efficiency of Si was significantly greater than that of ZrO₂ in this glass system.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

Crystallization of Li2O · Al2O3· 6SiO2 Glasses Containing Niobium Pentoxide as Nucleating Dopant

Niobium pentoxide (T form, orthorhombic system) was utilized to promote devitrification in Li₂O · Al₂O₃· 6SiO₂ glasses. Two or more mole percentage of this nucleating dopant enhanced crystallization in these glasses. Glasses containing 4.0 and 8.0 mol% T-Nb₂O₅ exhibited a high tendency to form dispersed TT-Nb₂O₅ (monoclinic system) precipitates during the glass quenching process. The crystallization process in glasses containing 2.0 or 4.0 mol% T-Nb₂O₅ occurred as microphase separation, followed by the formation of dispersed TT-Nb₂O₅ crystalline precipitates (760°C), followed by β-quartz solid-solution ( ss ) formation (850° to 900°C) heterogeneously nucleated from the precipitates. β-quartz( ss ) transformed to β-spodumene( ss ), along with a polymorphic transition from the TT-Nb₂O₅ to M-Nb₂O₅ (tetragonal system) crystalline phase.     

Poly(methylsilane)—A High Ceramic Yield Precursor to Silicon Carbide

Preceramic polymers offer exceptional potential for low-temperature processing of both oxide and non-oxide ceramics. In addition, shapes such as fibers, films, and membranes that are not commonly available using standard processing techniques are readity available using preceramic polymers. In non-oxide ceramics, the ceramic products generally available from preceramics do not exhibit all of the typical properties associated with the same materials produced by standard, high-temperature processing approaches. In part, this appears to be because there are very few preceramic polymers that lead to high-purity, single-phase materials. Poly(methylsilane), (–[MeHSi] _x –), produced from MeSiH₃, can be used to produce relatively pure, bulk SiC at temperatures below 1000°C. The transformation process from polymer to ceramic is followed by ²⁹Si NMR and diffuse reflectance IR. The polymer first undergoes a major rearrangement from poly(silane) to poly(carbosilane) at 400°C. Above 400°C, the resulting poly(carbosilane) decomposes to a hydrogenated form of SiC as shown by spectroscopic analysis of the 600°C material. Further heating, to 1000°C for 1 h, provides very narrow ²⁹Si peaks indicative of β-SiC mixed with small amounts of α-SiC polytypes. Chemical analysis, when coupled with the ²⁹Si and XRD results, suggests that poly(methylsilane) produces resonably pure, nanocrystalline SiC at temperatures much lower than previously observed for other SiC preceramic polymers.     

Hot Isostatic Press Sintering and Properties of Silicon Nitride without Additives

Fully densified silicon nitride without additives was fabricated by means of hot isostatic pressing. The sintering process of highly pure powder was investigated with special interest in the evolution of α–β phase transformation, densification, and microstructure development. It was observed that the transformation occurred without a liquid phase below 1730°C, which corresponds to the melting point of SiO₂. Above 1730°C, the densification and β-grain elongation accelerated concurrently because of the appearance of liquid SiO₂. However, full densification was attained at 1950°C together with marked grain growth. Flexural strength, microhardness, fracture toughness, and Young's modulus of sintered bodies were measured as a function of temperature. In the sintered body started from highly pure powder, excellent MOR behavior was found up to 1400°C. Impurity content of a few hundred ppm was found to be sufficient to make densification easy and to degrade high-temperature strength.     

Effect of Cr3C2 Addition on the Sintering of SiC-TiC Composite

The densification behavior and mechanical properties of SiC-30TiC (in volume percent) composites prepared with Cr₃C₂ additive were investigated. By hot-pressing a SiC-30TiC-lCr₃C₂ specimen at 1950°C, 98.5% of theoretical density was achieved and the specimen exhibited a fracture strength of 750 MPa. For the SiC-30TiC-10Cr₃C₂ specimen, (β-α transformation of SiC was observed to occur during hot-pressing and in situ growth of elongated α-SiC grains resulted in an increase of fracture toughness. Micro-structural observations using high-resolution TEM indi-cated that no liquid phase was present at the interfaces.     

The System SiO2–P2O2

Phase relations in the binary system between SiO₂-P₂O₅ and SiO₂ were investigated by the quenching method using sealed platinum tubes to prevent the loss of P₂O₅. The compound Si0₂-P₂O₅ exists in two forms, the low-temperature β form inverting sluggishly but reversibly to the high-temperature β form at 1030°C. The β form melts congruently at 1290°C. The compound 2SiO₂-P₂O₅ melts incongruently at 1120°C to a silica-rich liquid and SiO_a-P₂O₅. In the region between 5 and 25 mole % PO₂, reactions were so sluggish that no data could be obtained by quenching.     

Thermal Stability, Phase Evolution, and Crystallization in Si-B-C-N Ceramics Derived from a Polyborosilazane Precursor

Amorphous Si-B-C-N ceramic powder samples obtained by thermolysis of boron-modified polysilazane, {B[C₂H₄Si(H)NH]₃} _n , were isothermally annealed at different temperatures (1400–1800°C) and hold times (3, 10, 30, and 100 h). A qualitative and semiquantitative analysis of the crystallization behavior of the materials was performed using X-ray diffraction (XRD). The phase evolution was additionally followed by ¹¹B and ²⁹Si MAS NMR as well as by FT-IR spectroscopy in transmission and diffuse reflection (DRIFTS) modes. Bulk chemical analyses of selected samples were performed to determine changes in the chemistry/phase composition of the materials. It was observed that silicon carbide is the first phase to nucleate around 1400–1500°C, whereas silicon nitride nucleates at and above 1700°C. Crystallization accelerates with increasing annealing temperature and proceeds with increasing annealing time. Furthermore, the surface area of the powders strongly influences the thermal stability of silicon nitride and thus controls overall chemical and phase composition of the materials on thermal treatment.     

Chemical Durability of Silicon Oxycarbide Glasses

Silicon oxycarbide (SiOC) glasses with controlled amounts of Si—C bonds and free carbon have been produced via the pyrolysis of suitable preceramic networks. Their chemical durability in alkaline and hydrofluoric solutions has been studied and related to the network structure and microstructure of the glasses. SiOC glasses, because of the character of the Si—C bonds, exhibit greater chemical durability in both environments, compared with silica glass. Microphase separation into silicon carbide (SiC), silica (SiO₂), and carbon, which usually occurs in this system at pyrolysis temperatures of >1000°–1200°C, exerts great influence on the durability of these glasses. The chemical durability decreases as the amount of phase separation increases, because the silica/silicate species (without any carbon substituents) are interconnected and can be easily leached out, in comparison with the SiOC phase, which is resistant to attack by OH⁻ or F⁻ ions.     

Effect of Heat Treatment on Rapidly Quenched AgI-Based Silver Orthoborate Glasses Containing Large Amounts of AgI

The effect of heat treatment on the twin-roller rapidly quenched 75AgI·18.7Ag₂O6.3B₂O₃ glass was investigated by differential scanning calorimetry (DSC), high-temperature X-ray diffraction (XRD), and field-emission-type scanning electron microscopy. The glass had an inhomogeneous microstructure with dispersed particles 40-60 nm in diameter at room temperature. On the other hand, island regions of several hundred nanometers with fine dispersed particles about 20-30 nm in diameter were observed in the glass after heating to 120°C. DSC and high-temperature XRD measurements revealed that crystallization occurred at around 120°C, which is lower than the α-β phase transformation temperature (147°C), to form α-AgI in the glass. The crystallization of α-AgI from the glass below the α-ß phase transformation temperature strongly supports the possibility of the existence of α-AgI nuclei in AgI-based silver orthoborate glasses. Validating the existence of AgI microcrystals supports the microdomain model for superionic AgI-based glasses.     

Boron Redistribution in Sintered α-SiC During Thermal Oxidation

The redistribution of the boron impurity in sintered α-SiC during thermal oxidation was investigated over the temperature range 1200· to 1400,°C using sputter-induced photon spectrometry (SIPS). The process was modeled with the Stanford University Processing Engineering Models Program (SUPREM) which permitted the estimation of diffusivities of boron in the growing oxide and the substrate. The process is characterized by segregation of the boron in the oxide near the interface and a corresponding depletion of boron in the substrate. The apparent diffusivities in the oxide were about three orders of magnitude higher than the published values for boron in pure SiO₂, presumably because the film is much less pure than pure SiO₂. The apparent diffusivities of the boron in the polycrystalline silicon carbide were almost five orders of magnitude higher than the published values for boron in single-crystal silicon carbide. The diffusivities in the silicon carbide represent boron transport via the grain boundaries which were partially oxidized during the thermal treatment.     

Differential Thermal Calorimetric Determination of the Thermodynamic Properties of Kaolinite

Four samples of kaolinite were investigated to determine the exothermic reaction enthalpy by differential thermal calorimetry. The measured 9 kcal/mol for the 980°C exothermic reaction enthalpy corresponds to the calculated heat of crystallization of silica at this temperature. Literature evidence discounts the crystallization of the other participating phases, mullite and silicon spinel. An NaOH extraction technique was used to remove the amorphous silica from a kaolinite fired at 850°C; this extraction removed the 980°C exotherm. It is tentatively suggested, therefore, that most of the heat release at 980°C on firing kaolinite accompanies the reaction SiO₂(amorphous) → SiO₂(β-quartz).     

Nanostructure and Micromechanical Properties of Silica/Silicon Oxycarbide Porous Composites

The microhardness–nanostructure correlation of a series of silica/silicon oxycarbide porous composites has been investigated, as a function of pyrolysis temperature, T _p. The pyrolyzed products have been studied by means of scanning electron microscopy, mercury porosimetry, chemical analysis, solid-state ²⁹Si-NMR, X-ray diffraction, Raman spectroscopy, and microindentation hardness. Two distinct regimes are found for the microhardness behavior with T _p. In the low-temperature regime (1000°C ≤ T _p < 1300°C), the material response to indentation seems to be dominated by the large amount of pores present in the samples. In this T _p range, low microhardness values, H , are found (<110 MPa). Above T _p= 1300°C, a conspicuous H increase is observed. In this high-temperature regime ( T _p= 1300–1500°C), microhardness values are shown to notably increase with increasing pyrolysis temperature. The H behavior at T _p= 1300–1500°C is discussed in terms of (i) the volume fraction of pores and the average pore size, (ii) the bond density of the oxycarbide network, and (iii) the occurrence of a nanocrystalline SiC phase.     

Synthesis of Al4SiC4

The conditions necessary for synthesizing Al₄SiC₄ from mixtures of aluminum, silicon, and carbon and kaolin, aluminum, and carbon, as starting materials, were examined in the present study. The standard Gibbs energy of formation for the thermodynamic reaction SiC( s ) + Al₄C₃( s ) = Al₄SiC₄( s ) changed from positive to negative at 1106°C. SiC and Al₄C₃ formed as intermediate products when the mixture of aluminum, silicon, and carbon was heated in argon gas, and Al₄SiC₄ then formed by reaction of the SiC and Al₄C₃ at >1200°C. Al₄C₃, SiO₂, Al₂O₃, SiC, and Al₄O₄C formed as intermediate products when the mixture of kaolin, aluminum, and carbon was heated under vacuum, and Al₄SiC₄ formed from a reaction of those intermediate products at >1600°C.     

Radiation Effects on the Physical Properties of Low-Expansion-Coefficient Glasses and Ceramics

The effects of irradiation on the coefficient of thermal expansion (CTE), the density, and the elastic moduli of low-CTE materials such as amorphous SiO₂, ultra-low-expansion glass, β-SiC, Astrositall, and Zerodur were surveyed. It was found that the properties of all of these materials were affected by radiation exposures up to 2 × 10⁹ rd; SiO₂ and SiC were the most radiation-resistant of the materials studied.     

Crystallization of MgO-CaO-SiO2-P2O5 Glass

The crystallization behavior of a glass with a composition of 40 wt% 3CaO · P₂O₅−60 wt% CaO · MgO · 2SiO₂ was investigated. The primary crystalline phase was apatite with a dendritic form and ellipsoidal shape. β-(3CaO · P₂O₅) and CaO · MgO · 2SiO₂ were crystallized as samples heated to 990°C, and a three-layer structure was obtained. The development and morphology of this construction were explained by both the surface crystallization of the apatite and CaO · MgO · 2SiO₂ and the bulk crystallization of apatite and the CaO · MgO · 2SiO₂-β-(3CaO · P₂O₅) composite.     

Wollastonite from Hydrothermally Synthesized Calcium Hydrometasilicate Obtained from Various Modifications of Silica

A hydrothermally CaO–SiO₂–H₂O system was investigated at 150°–200°C, 2.5 h (CaO:SiO₂=0.95) using various modifications of SiO₂ in the presence of a mineralizer. Synthetic (stabilized) γ-tridymite is the most reactive among SiO₂ modifications. In the reaction mixture, the optimal concentration of the mineralizer (KOH) is 2% (versus the solid phase). The binding degree of CaO with SiO₂ practically is 100% at 150°C. It is impossible to synthesize CSH free of C₂SH on the basis of β-cristobalite and β-quartz under the investigation conditions without the use of mineralizer. The calorimetric effects as well as heat of de-hydration of hydrosilicates were determined during their transformation into wollastonite. The entropy change at the peaks has been calculated.     

Preparation and Sintering of a Porous Glass-Ceramic in the System Na2 O-B2O3-Ga2O3

Clear glasses were produced when SiO₂ in sodium borosilicate glasses was replaced by Ga₂O₃. Phase separation and/or crystallization occurred after heat treatment. The porous skeleton of leached material consisted of monoclinic β-Ga₂O₃. The specific surface areas and pore radii are comparable to those of porous SiO₂. Alumina contamination influenced the structure of the porous product.